Electrical discharge machining is used in the manufacture of various metallic components, including for example, gas turbine engine components such as turbine airfoils. EDM uses high energy electrical discharges (i.e., sparks) between an electrode and an electrically conductive workpiece to remove material from the workpiece. The electrode is advanced to the workpiece so as to be separated by only a small distance or gap. A dielectric fluid medium fills the gap and a differential voltage of specified magnitude is applied between the electrode and the workpiece causing the dielectric medium to ionize and break down. Current then starts to flow between the electrode and the workpiece and through the dielectric medium. The current causes heat to be generated at the surface of the workpiece resulting in a significant temperature rise and localized melting of the workpiece material. The magnitude of the differential voltage is reduced, the dielectric medium de-ionizes and the current terminates. Heat generation ceases thereby allowing the electrode and the workpiece to cool somewhat. The molten material solidifies as it is flushed from the work area by the dielectric medium, leaving a crater in the workpiece. The crater typically has a shape complementary to that of the electrode. This process, or cycle, commonly referred to as an "on/off" cycle is repeated in a pulsating manner until the desired machining of the workpiece is complete.
EDM is particularly useful for precision drilling and is used to drill arrays of diffused cooling holes in airfoils for gas turbine engines as disclosed for example in Sidenstick, U.S. Pat. No. 4,197,443 issued Apr. 8, 1980 and entitled Method And Apparatus For Forming Diffused Cooling Holes In An Air Foil. In gas turbine engines, it is common practice to cool the surface of airfoils such as turbine airfoils by passing high pressure air through channels in the airfoil and out through holes in the wall of the airfoil thereby providing a layer or a film of cooling gas over the airfoil surface. It has been found that film cooling effectiveness can be obtained with reduced amounts of cooling air by the use of diffusion film holes which have enlarged openings. Such film holes must be precisely formed and positioned in the airfoil.
The precision drilling of such air holes is achieved with a multi-tooth electrode such as that disclosed in Cross et al., U.S. Pat. No. 4, 922,076 issued May 1, 1990 entitled Electro-Discharge Machining Electrode. Such electrodes are formed from thin copper ribbon or other malleable conductive material and have an array of teeth for forming a corresponding array of air holes in the airfoil. The electrodes are subject to burrs, slivers and bending in the manufacturing process. Further, the teeth are quite thin and consequently subject to bending or deformation from handling. For example, the teeth in the electrode disclosed in Cross et al., have a leading section which is typically only 0.005"-0.030" in diameter.
In order to achieve the necessary precision drilling, the electrode must be within specified configuration and dimension tolerances. In addition, the electrode must be properly aligned relative to the airfoil. If the electrode is out of tolerance or improperly aligned relative to the airfoil, the airfoil will be unacceptable for use and scrapped which results in increased manufacturing costs.
EDM machines are commercially available that provide automated drilling of airfoils. It would be desirable to have integrated electrode testing for configuration and dimension tolerances and for alignment in an automated drilling process. It would also be desirable to have a testing apparatus that can be retrofitted to existing EDM machines.